The present invention relates to a data transmission system for performing data transmission or reception between a personal computer and an ECR (Electronic Cash Register).
FIG. 1 shows a system configuration of a conventional data communication system. A plurality of ECR1, ECR2, . . . are connected to a personal computer (to be referred to as a PC hereinafter) through in-lines. In this data communication system, the PC is operated according to a flow in FIG. 2. The flow includes communications control processing (i.e., a protocol sequence) between the respective ECRs, i.e., ECR1, ECR2, . . . and a PC application sequence (more specifically, writing into floppy disk FDD coupled to the PC). When this flow is started, one unit data (i.e., one-block data) is received by an input buffer in interface I/F (step SA). If data is normally received, the PC responds to the ECR (step SB). Reception data is character-converted, and the resultant data is written in disk FDD (steps SC and SD). The CPU determines in step SE whether the reception data is the last block data. The data reception is repeated until the data is the last block data.
In the data communication system described above, at the end of the protocol sequence, the next processing (settlement processing after the last block data is sent out from the ECR side) is started at the ECR side. However, the transmitted data has not yet been written in the floppy disk at the PC side. In this case, if a power failure or a voltage drop occurs at the PC side at the end of the protocol sequence, the reception data is lost before it is completely written in the floppy disk Even if the PC requests retransmission of the lost data to the ECR side, the ECR has started the next processing (e.g., settlement processing). Therefore, the requested data cannot be sent from the ECR to the PC.